There is only one bus company offering a service from Lisbon to Alvor.
this is: http://www.renex.pt
If you are arriving at 18:00, by the time you clear customs, immigration, collect your bags and get a cab to the bus station, the only suitable bus will be the departure at 21:00 arriving 01:05.
This bus departs from Lisbon Gare Oriente Bus Station.
You can take the Aeroshuttle from outside the airport terminal to Gare Oriente.(CARRIS Nº96) - Available everyday, every 30 minutes between 7am and 11pm, the aeroshuttle connects the Airport with Gare Oriente.
The bus will stop in the centre of Alvor but at 01:05 there may be few taxis around.
